a11y: Use gtk_tree_view_get_n_columns()
a11y: Use gtk_tree_view_get_n_columns()

In fact, invent our own function get_n_columns() which actually only
counts the visible functions, because that's the only ones we care
about.

The places where it's not used yet will be changed in the coming
patches.

Lyude/gtk-,Adamovskiy/gtk,ebassi/gtk,Lyude/gtk-,jigpu/gtk,bratsche/gtk-,bratsche/gtk-,msteinert/gtk,ebassi/gtk,davidt/gtk,davidgumberg/gtk,Sidnioulz/SandboxGtk,jadahl/gtk,bratsche/gtk-,ahodesuka/gtk,bratsche/gtk-,alexlarsson/gtk,davidgumberg/gtk,davidgumberg/gtk,ebassi/gtk,davidgumberg/gtk,jigpu/gtk,jessevdk/gtk,bratsche/gtk-,davidt/gtk,jadahl/gtk,davidt/gtk,grubersjoe/adwaita,ebassi/gtk,Distrotech/gtk2,davidgumberg/gtk,Adamovskiy/gtk,alexlarsson/gtk,jigpu/gtk,jadahl/gtk,ebassi/gtk,jigpu/gtk,Sidnioulz/SandboxGtk,Adamovskiy/gtk,alexlarsson/gtk,Lyude/gtk-,davidt/gtk,davidgumberg/gtk,davidt/gtk,davidgumberg/gtk,msteinert/gtk,msteinert/gtk,chergert/gtk,Lyude/gtk-,jessevdk/gtk,grubersjoe/adwaita,ebassi/gtk,jadahl/gtk,jadahl/gtk,bratsche/gtk-,grubersjoe/adwaita,Distrotech/gtk2,chergert/gtk,jigpu/gtk,grubersjoe/adwaita,Distrotech/gtk2,Sidnioulz/SandboxGtk,msteinert/gtk,grubersjoe/adwaita,jadahl/gtk,chergert/gtk,jigpu/gtk,ahodesuka/gtk,Lyude/gtk-,msteinert/gtk,Lyude/gtk-,ahodesuka/gtk,chergert/gtk,jessevdk/gtk,ahodesuka/gtk,alexlarsson/gtk,jessevdk/gtk,Lyude/gtk-,alexlarsson/gtk,Lyude/gtk-,Distrotech/gtk2,chergert/gtk,Sidnioulz/SandboxGtk,Adamovskiy/gtk,Adamovskiy/gtk,jessevdk/gtk,ahodesuka/gtk,davidt/gtk,grubersjoe/adwaita,chergert/gtk,chergert/gtk,alexlarsson/gtk,Sidnioulz/SandboxGtk,grubersjoe/adwaita,Distrotech/gtk2,Sidnioulz/SandboxGtk,alexlarsson/gtk,davidgumberg/gtk,Adamovskiy/gtk,jigpu/gtk,msteinert/gtk,ahodesuka/gtk,jadahl/gtk,Adamovskiy/gtk,jessevdk/gtk,jadahl/gtk,Adamovskiy/gtk,grubersjoe/adwaita,Distrotech/gtk2,alexlarsson/gtk,jessevdk/gtk,ahodesuka/gtk,jigpu/gtk,ahodesuka/gtk,chergert/gtk